This invention relates to a lash adjuster mounted in an engine valve operating mechanism.
Many known valve operating mechanisms include an arm that pivots corresponding to the rotation of a cam, thereby pushing a valve stem connected to a valve provided at an intake or exhaust port of an engine and moving the valve.
When such a valve operating mechanism is thermally expanded due to increased ambient temperature while the engine is running, due to differences in thermal expansion coefficient between component parts of the valve operating mechanism, gaps between the respective component parts of the valve operating mechanism (including the gap between the arm and the valve stem) may change, thus producing noise and causing the valve to be opened and closed at wrong timing. Also, when the sliding portions of the valve operating mechanism become worn, gaps between the component parts of the valve operating mechanism (such as the gap between the valve and the valve seat) may change, thus producing noise.
In order to eliminate such changes in gaps between the component parts of the valve operating mechanism, the pivot point of the arm is, in many cases, supported by a lash adjuster to automatically adjust the pivot point of the arm with the lash adjuster.
One known lash adjuster of this type includes a cylindrical housing with an open top, a screw rod having an external thread formed on its outer periphery and in threaded engagement with an internal thread formed on the inner periphery of the housing, and a spring biasing the screw rod outwardly of the housing, with the protruding end of the screw rod supporting the pivot point of the arm.
The external thread formed on the outer periphery of the screw rod and the internal thread formed on the inner periphery of the housing both comprises a pressure flank that receives pressure when a force that tends to push the screw rod into the housing (hereinafter referred to as “pushing force”) is applied, and a clearance flank having a smaller flank angle than the pressure flank.
With this lash adjuster, when pushing force is applied to the screw rod due to the rotation of the cam, the pressure flank of the external thread formed on the outer periphery of the screw rod is supported by the pressure flank of the internal thread formed on the inner periphery of the housing, so that the screw rod is not axially movable. When the relative position between the arm and the valve stem changes due e.g. to thermal expansion of the valve operating mechanism, the screw rod moves axially in the housing while rotating, thereby eliminating the changes in gaps between component parts of the valve operating mechanism.
In this arrangement, the moment the pressure flank of the external thread formed on the outer periphery of the screw rod is supported on the pressure flank of the internal thread formed on the inner periphery of the housing, any engine oil that exists between the pressure flanks may form an oil film due to the squeezing effect. Especially while the ambient temperature is low, an oil film tends to form between the pressure flanks because the viscosity of engine oil is high at low temperature.
If an oil film forms between the pressure flanks, the oil film dramatically reduces the friction between the pressure flanks. Thus, when pushing force is applied to the screw rod due to the rotation of the cam, the screw rod may rotate and be pushed into the housing. If the screw rod is pushed into the housing, the pivot point of the arm moves, thus causing the valve to be impulsively seated on the valve seat and producing noise.
In order to expel such oil film between the pressure flanks, a lash adjuster is proposed in which oil film expelling grooves are formed in the pressure flank of the external thread formed on the outer periphery of the screw rod so as to extend parallel to the external thread (JP3-501758A). For the same purpose, in another lash adjuster, axial grooves are formed by broaching in the internal thread formed on the inner periphery of the housing to divide the internal thread into a plurality of separate portions (JP2000-130114A). In these lash adjusters, engine oil present between the pressure flanks is expelled into the oil film expelling grooves, thereby preventing formation oil film between the pressure flanks.
In the case of the former lash adjuster, because the oil film expelling grooves are formed parallel to the thread, the oil film expelling grooves are long, so that engine oil in the oil film expelling grooves is difficult to flow. This in turn makes it difficult for engine oil present between the pressure flanks to flow into the oil film expelling grooves, and thus making it difficult to effectively prevent formation of oil film between the pressure flanks.
For the latter lash adjuster, because the internal thread is formed by a tap and the oil film expelling grooves are formed by broaching, a larger number of manufacturing steps are needed, and thus the manufacturing cost is high. Also, if the internal thread is axially long, it is difficult to form oil film expelling grooves by broaching.
An object of the present invention is to provide a lash adjuster which can effectively eliminate oil film between the pressure flanks and which can be manufactured at a low cost.